'\" et
.TH PTHREAD_CREATE "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
pthread_create
\(em thread creation
.SH SYNOPSIS
.LP
.nf
#include <pthread.h>
.P
int pthread_create(pthread_t *restrict \fIthread\fP,
    const pthread_attr_t *restrict \fIattr\fP,
    void *(*\fIstart_routine\fP)(void*), void *restrict \fIarg\fP);
.fi
.SH DESCRIPTION
The
\fIpthread_create\fR()
function shall create a new thread, with attributes specified by
.IR attr ,
within a process. If
.IR attr
is NULL, the default attributes shall be used. If the attributes
specified by
.IR attr
are modified later, the thread's attributes shall not be affected.
Upon successful completion,
\fIpthread_create\fR()
shall store the ID of the created thread in the location referenced by
.IR thread .
.P
The thread is created executing
.IR start_routine
with
.IR arg
as its sole argument. If the
.IR start_routine
returns, the effect shall be as if there was an implicit call to
\fIpthread_exit\fR()
using the return value of
.IR start_routine
as the exit status. Note that the thread in which
\fImain\fR()
was originally invoked differs from this. When it returns from
\fImain\fR(),
the effect shall be as if there was an implicit call to
\fIexit\fR()
using the return value of
\fImain\fR()
as the exit status.
.P
The signal state of the new thread shall be initialized as follows:
.IP " *" 4
The signal mask shall be inherited from the creating thread.
.IP " *" 4
The set of signals pending for the new thread shall be empty.
.P
The thread-local current locale
and the alternate stack
shall not be inherited.
.P
The floating-point environment shall be inherited from the creating
thread.
.P
If
\fIpthread_create\fR()
fails, no new thread is created and the contents of the location
referenced by
.IR thread
are undefined.
.P
If _POSIX_THREAD_CPUTIME is defined, the new thread shall have a
CPU-time clock accessible, and the initial value of this clock shall
be set to zero.
.P
The behavior is undefined if the value specified by the
.IR attr
argument to
\fIpthread_create\fR()
does not refer to an initialized thread attributes object.
.SH "RETURN VALUE"
If successful, the
\fIpthread_create\fR()
function shall return zero; otherwise, an error number shall be
returned to indicate the error.
.SH ERRORS
The
\fIpthread_create\fR()
function shall fail if:
.TP
.BR EAGAIN
The system lacked the necessary resources to create another thread, or
the system-imposed limit on the total number of threads in a process
{PTHREAD_THREADS_MAX}
would be exceeded.
.TP
.BR EPERM
The caller does not have appropriate privileges to set the required
scheduling parameters or scheduling policy.
.P
The
\fIpthread_create\fR()
function shall not return an error code of
.BR [EINTR] .
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
There is no requirement on the implementation that the ID of the
created thread be available before the newly created thread starts
executing. The calling thread can obtain the ID of the created thread
through the
.IR thread
argument of the
\fIpthread_create\fR()
function, and the newly created thread can obtain its ID by a call to
\fIpthread_self\fR().
.SH RATIONALE
A suggested alternative to
\fIpthread_create\fR()
would be to define two separate operations: create and start. Some
applications would find such behavior more natural. Ada, in
particular, separates the ``creation'' of a task from its
``activation''.
.P
Splitting the operation was rejected by the standard developers for
many reasons:
.IP " *" 4
The number of calls required to start a thread would increase from one
to two and thus place an additional burden on applications that do not
require the additional synchronization. The second call, however,
could be avoided by the additional complication of a start-up state
attribute.
.IP " *" 4
An extra state would be introduced: ``created but not started''. This
would require the standard to specify the behavior of the thread
operations when the target has not yet started executing.
.IP " *" 4
For those applications that require such behavior, it is possible to
simulate the two separate steps with the facilities that are currently
provided. The
\fIstart_routine\fR()
can synchronize by waiting on a condition variable that is signaled by
the start operation.
.P
An Ada implementor can choose to create the thread at either of two
points in the Ada program: when the task object is created, or when
the task is activated (generally at a ``begin''). If the first
approach is adopted, the
\fIstart_routine\fR()
needs to wait on a condition variable to receive the order to begin
``activation''. The second approach requires no such condition
variable or extra synchronization. In either approach, a separate Ada
task control block would need to be created when the task object is
created to hold rendezvous queues, and so on.
.P
An extension of the preceding model would be to allow the state of the
thread to be modified between the create and start. This would allow
the thread attributes object to be eliminated. This has been rejected
because:
.IP " *" 4
All state in the thread attributes object has to be able to be set for
the thread. This would require the definition of functions to modify
thread attributes. There would be no reduction in the number of
function calls required to set up the thread. In fact, for an
application that creates all threads using identical attributes, the
number of function calls required to set up the threads would be
dramatically increased. Use of a thread attributes object permits the
application to make one set of attribute setting function calls.
Otherwise, the set of attribute setting function calls needs to be made
for each thread creation.
.IP " *" 4
Depending on the implementation architecture, functions to set thread
state would require kernel calls, or for other implementation reasons
would not be able to be implemented as macros, thereby increasing the
cost of thread creation.
.IP " *" 4
The ability for applications to segregate threads by class would be
lost.
.P
Another suggested alternative uses a model similar to that for process
creation, such as ``thread fork''. The fork semantics would provide
more flexibility and the ``create'' function can be implemented simply
by doing a thread fork followed immediately by a call to the desired
``start routine'' for the thread. This alternative has these
problems:
.IP " *" 4
For many implementations, the entire stack of the calling thread would
need to be duplicated, since in many architectures there is no way to
determine the size of the calling frame.
.IP " *" 4
Efficiency is reduced since at least some part of the stack has to be
copied, even though in most cases the thread never needs the copied
context, since it merely calls the desired start routine.
.P
If an implementation detects that the value specified by the
.IR attr
argument to
\fIpthread_create\fR()
does not refer to an initialized thread attributes object, it is
recommended that the function should fail and report an
.BR [EINVAL] 
error.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIfork\fR\^(\|)",
.IR "\fIpthread_exit\fR\^(\|)",
.IR "\fIpthread_join\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.12" ", " "Memory Synchronization",
.IR "\fB<pthread.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
